1. Field of the Invention
This invention relates to a traction control system for a vehicle which reduces the torque transmitted to the road surface from the driving wheels of the vehicle in order to suppress slip of the driving wheels.
2. Description of the Prior Art
There has been known a traction control system for a vehicle which reduces the torque transmitted to the road surface from the driving wheels of the vehicle in order to suppress slip of the driving wheels.
In the traction control system, the rate of slip of the driving wheels is detected and the driving force (torque transmitted to the road surface from the driving wheels) is reduced by lowering the engine output and/or applying brakes to the driving wheels so that the rate of slip of the driving wheels converges on a target rate of slip which is determined according to the friction coefficient of the road surface, thereby preventing deterioration in the starting performance or accelerating performance due to slip of the driving wheels caused by an excessive driving torque during starting or acceleration.
When the driving wheels slip during turning, it becomes difficult for the vehicle to make a stable turn. Especially it becomes more difficult for the vehicle to make a stable turn when the vehicle comes into an oversteering state in the case of a rear-drive vehicle and into an understeering state in the case of frontdrive vehicle.
Thus there has been proposed a traction control system in which when a rear-drive vehicle is in an oversteering state, the target rate of slip is reduced so that the traction control is initiated earlier, thereby ensuring stability during a turn. See, for instance, Japanese Unexamined Patent Publication No. 3(1991)-157255.
However, conventionally, whether the vehicle is in an instable state during turning, e.g., in an oversteering state (in the case of a rear-drive vehicle) or in an understeering state (in the case of a front-drive vehicle) cannot be constantly detected precisely and accordingly, when it is misjudged that the vehicle is in an instable state and the traction control is initiated earlier in spite of the fact that the vehicle is actually in a stable state, the engine output is unnecessarily lowered or the braking force is unnecessarily increased.
For example, in one known method of judging whether the vehicle is in an oversteering state, it is judged that the vehicle is in an oversteering state when the difference between the actual yaw rate of the vehicle and a reference yaw rate (a yaw rate which is to act on the vehicle on the basis of the vehicle speed, the turning angle of the steering wheel and the like) is larger than a predetermined value. This method gives rise to the following problem. That is, if the actual yaw rate is detected by a known yaw rate sensor, the yaw rate sensor instantaneously detects a large yaw rate, for instance, when one of the driven wheels runs into a ditch or runs onto snow during straight running. As a result, it is misjudged that the vehicle is in an oversteering state and the target rate of slip can be reduced in vain.
In the case where the actual yaw rate is calculated on the basis of the difference in the wheel speed between the left and right driven wheels and the vehicle speed, a large wheel speed difference is detected, for instance, when one of the driven wheels runs into a ditch or runs onto snow during straight running and the calculated value of the yaw rate becomes large though the yaw rate is actually small. As a result, it is misjudged that the vehicle is in an oversteering state and the target rate of slip can be reduced in vain.
Accordingly it is preferred that the traction control system, in which whether the vehicle is in an oversteering state is determined on the basis of the difference between the actual yaw rate and the reference yaw rate and the target rate of slip is reduced when it is determined that the vehicle is in an oversteering state, be provided with a means for avoiding an unnecessary reduction of the driving force due to a misjudgment of the oversteering state.
Further when the actual yaw rate increases during turning and the vehicle comes to an oversteering state, the driver sometimes turns the steering wheel in the counter direction to prevent spin of the vehicle and the steering wheel can be returned to the neutral position. In such a case, since the traction control has been initiated with the target rate of slip set at a value corresponding to the oversteering state, the target rate of slip is changed to a value corresponding to the straight running state as soon as the steering wheel is returned to the neutral position and it is determined that the vehicle is running straight, whereby the traction force is quickly increased and the running stability of the vehicle deteriorates.